**4.5 Heavy metals inhibition**

Heavy metals are present in various types of wastewater, including agro-industrial wastewater, landfill leachate and cane vinasses (Del Real et al., 2009; Yusof et al., 2009). Although many metals are required in trace amounts to provide sufficient growth to methanogens, the methanogenic activity in anaerobic reactors is strongly affected by excess amounts of heavy metals (Colussi et al., 2009). The toxic effects of metals in biological process is particularly due to the inhibition of enzymes activity as a result of metals binding to the SH group of the enzyme. The inhibitory concentrations of four heavy metals on methane-producing granular sludge that caused 50% reduction in cumulative methane production was found to be 7.5 mg/L of Zn, 27 mg/L of Cr, 35 mg/L of Ni and 36 mg/L of Cd with an order of Zn>Cr>Ni≈Cd (Altas, 2009). Whereas a different study revealed that 50% reduction in methane production occurred at 6.4 mg/L of Cu (II), 4.4 mg/L of Cd(II) and 18.0 mg/L of Cr(VI) with an order of Cd(II)>Cu(II)>Cr(VI) in anaerobic digestion of cattail with rumen culture (Yue et al., 2007).

Yue and coworker in 2007, indicated that metals cause anaerobic system failures when they are in the form of free ions (in its soluble form) and above certain concentrations (Table 3). The differences reported in the metals inhibitory concentration might be due to the several factors including variation in sludge characteristics, chemical form of heavy metals and microbial resistance to metals (Altas, 2009). Various heavy metals presence in wastewater

1950):

**6. Reactor types** 

Biogas Production from Anaerobic Treatment of Agro-Industrial Wastewater 103

The volumetric measurement of biogas must compensate the pressure and temperature differences. The equation 10, illustrate a simple method of gas volume compensation for a saturated gas taking into account the adjustment by pressure and temperature (Salisbury,

V� = V ∗ 17.626 ∗ �����

where *V* is the observed volume, *Vs* volume at standard conditions (60°F and 30 inches Hg), *H* is absolute gas pressure (inches Hg), *A* water vapor pressure (inches Hg), and *T* temperature of gas (°F). Pure methane at standard temperature and pressure has a lower heating value of 912 BTU/ft3 (34 kJ/L). Typical biogas of 65% methane has a heating value of approximately 600 BTU/ft3 (22.36 kJ/L), since only the methane portion will burn,

> **Biogas with 65% of methane (1000 L)**  600 L of natural gas 25.0 L of propane 22.3 L of Butane 17.79 L of gasoline 16.28 L of diesel

Many reactor configurations are used for the anaerobic treatment of agro-industrial wastewaters. Among them, the most common types are discussed and illustrated in Fig. 2.

Fig. 2. Most commonly used anaerobic reactors types: (A) Completely mixed anaerobic digester, (B) UASB reactor, (C) AFB reactor, (D) Upflow AF reactor (Ersahin et al., 2011)

The completely stirred anaerobic digester (CSTR) is the basic anaerobic treatment system with an equal hydraulic retention time (HRT) and solids retention time (SRT) in the range of 15-40 days in order to provide sufficient retention time for both operation and process stability. Completely mixed anaerobic digesters without recycle are more suitable for wastes with high solids concentrations (Tchobanoglous et al., 2003). A disadvantage of this system

approximate equivalents of biogas to others fuels are presented in the table 5.

Table 5. Equivalents of biogas others fuels (Palmer, 1981)

**6.1 Completely stirred anaerobic digester** 

����.���� (10)

also showed synergistic effects during anaerobic treatment process. For instance, the presence of chromium in the sludge results in higher toxicity of copper (Colussi et al., 2009).

Altas in 2009 showed that low concentrations of metals in the anaerobic reactor can be extremely toxic. Meanwhile, Cantrell and coworkers (2008) indicated that high concentrations of soluble metals have come to completely stop the production of biogas in an anaerobic system. To combat metal toxicity in the anaerobic degradation process, they can be precipitated as sulphate salts and carbonate salts, except iron and chromium.


Table 3. Concentrations of inorganic compounds inhibitory of anaerobic process (Yue *et al.*, 2007).
